Terrain Modeling is important when you are simulating the behavior of a wireless network, and I agree there is a challenge finding good terrain data..
Did you check out the sources listed in the How to Find and Install Terrain Data in the Terrain Modeling Module (TMM) > Getting Started. (https://support.riverbed.com/bin/support/static/oc1sg8lar130eg7se34pvjhkdg//doc/opnet/OnlineHTML/Modeler/modeler_17.5_PL…)
Note the link assumes you have access to Riverbed's support website.
The above page also describes all the Terrain Data File formats supported. Although OpenFlight and MetaFlight are supported, they require an additional 3DNV license.
Newer versions of Riverbed Modeler, allow you to develop alternative ways to provide terrain data. As I recall, in v18+ there is a lab describing how you can dynamically generate terrain data.
Good luck with your modeling,
One of the best places to get terrain data files for use in Modeler is the USGS EarthExplorer website at http://earthexplorer.usgs.gov. The data is free, available in a number of formats, and covers most of the Earth. We generally use the Digital Elevation SRTM 1 Arc-Second files in Modeler.